Benomyl and its metabolite carbendazim are benzimidazole fungicides which cause testicular toxicity in rats at dose levels where no other organ system is affected. A recent study has supplied strong evidence that carbenazim (CBZ), and not benomyl, is responsible for testicular damage. Histopathologically, testicular toxicity after CBZ administration is characterized by a stage specific sloughing of the seminiferous epithelium. CBZ is known to inhibit microtubule assembly but the relationship of this to the cellular events which underlie sloughing is not known. The hypothesis central to the present proposal is that the singular sensitivity of the testis to CBZ toxicity is the result of disruption of a specific microtubule associated protein (MAP)/tubulin interaction only found in the testis with resultant loss of microtubule assembly and decreased phosphorylation of MAP or tubulin protein(s). A model is presented which addresses the role of testis microtubule vs. intermediate filament structures in the stage specific sloughing induced by CBZ. The first specific aim is to establish that the effect of CBZ on microtubule assembly is specific for testis microtubule assembly and involves disruption of a testis specific MAP-tubulin interaction. The second specific aim is to identify the MAP/tubulin testis-specific proteins which are affected by CBZ and address the possibility that CBZ induced changes in protein phosphorylation play a role in cytoskeletal collapse. The final specific aim will determine the relationship to sloughing of early cytoskeletal changes induced by CBZ administered at dose levels both above and below the sloughing threshold in normal rats and vim-knockout mice. Preliminary immunocytochemical data has indicated that, concomitant with loss of microtubule structure, intermediate filaments collapse towards the basal membrane. A model is proposed which explains the stage specific effects of CBZ in terms of the function of microtubules and intermediate filaments at different stages of germ cell development. The vimentin-knockout mouse provides an excellent experimental model for testing the role of intermediate filaments in the sloughing events which follow CBZ administration.